The invention relates to the reproduction of masters or originals.
Masters or originals in the form of photographs frequently exhibit large variations in brightness. Paper copies or prints of such masters are overexposed in the light areas or underexposed in the dark areas. As a result, details and fine structures are poorly visible, or no longer visible, in the copies.
To improve the quality of a copy made from a master with large variations in brightness, the German Patent Publication No. 40 40 498 proposes to mask the master with an unsharp mask. The unsharp mask reduces the overall contrast while providing greater contrast for the details. This is different from a sharp mask which makes the edges sharper but decreases the contrast for the details. The German Patent Publication teaches a mask with very low resolution, i.e., a mask which produces a very unsharp image of the master.
It has been found that the method of the German Patent Publication normally yields outstanding results but that undesirable effects can occur with certain pictures. For instance, if a picture contains a dark scene against a background of medium density, the high degree of unsharpness of the mask can result in a very bright border which extends around the dark scene and blends into the background only at a relatively great distance from such scene. Due to its somewhat singular shape, this border is referred to as a "white halo". A corresponding "black halo" can arise around a bright scene set against a background of medium density.